The present invention relates to a steering gear for turning steerable wheels of a vehicle and, in particular, to a power assist rack and pinion steering gear.
There are many known power rack and pinion steering gear designs. Some use hydraulic power and others electrical power. Power rack and pinion steering gear designs which utilize electrical power normally include an electric motor. An example is shown in U.S. Pat. No. 3,983,953. In U.S. Pat. No. 3,983,953 the electric motor is energized in response to turning of the steering wheel. When the steering wheel turns a switch is actuated to actuate the motor. The motor applies a driving force to the pinion. Thus, the motor provides an assist to the pinion.
The present invention is directed to an electric power rack and pinion steering gear. Specifically, the present invention includes a power transmitting member which is movable linearly in opposite directions to effect turning of the wheels of the vehicle in opposite directions. A first portion of the member comprises a rack. A manually rotated pinion engages the rack and applies a force to the rack. An electric motor is provided to apply an assisting force to a second portion of the member. The force is applied directly to the power transmitting member, but not through the pinion.
The electric motor has a rotatable armature encircling the force transmitting member. The armature has a drive connection to the member so as to effect linear movement of the member in a desired direction in response to rotation of the armature.
In the preferred embodiment, the drive connection is located between an armature and a linearly movable member and includes a ball nut. A screw thread is formed on the second portion of the member. Rotation of the armature is converted into linear movement of the member through the ball nut and screw thread.
The preferred embodiment includes a torsion bar located in the steering column between the steering wheel and the pinion. The torsion bar deflects in response to the torque created by rotation of the steering wheel. A torsion bar deflection sensor is provided to create an electric signal. If there is a sufficient torque, i.e., resistance to turning of the pinion, the torsion bar will twist, upon rotation of the steering wheel and the electric signal will be created to effect energization of the motor to provide an assist to the force transmitting member.
If the electric motor fails, the pinion is rotated manually to effect movement of the rack. Manual steering of the vehicle will thus be possible. Thus, a manual failsafe mode of operation of the steering gear is provided.
In one specific embodiment of the present invention, the armature of the electric motor is mounted coaxially with the screw portion of the force transmitting member which moves linearly. The armature is axially fixed relative to the member. With current flows through the armature, the armature rotates. The armature is drivingly connected to the ball nut. Rotation of the armature causes rotation of the ball nut. A plurality of balls which are associated with the ball nut and the screw portion of the member effect linear movement of the force transmitting member in response to rotation of the ball nut.
In another embodiment of the present invention, a clutch is interposed between the armature and the movable force transmitting member. Specifically, in this embodiment, the armature of the motor is axially offset in a magnetic field created by permanent magnets of the motor. When an electrical current is applied to the armature, the armature tends to center itself in the magnetic field of the permanent magnets by moving axially in the magnetic field. When the armature moves axially, the clutch is engaged to create a drive connection between the armature and the ball nut. Thus, when the ball nut is rotated by rotation of the armature, the member moves axially through engagement of the balls in the screw portion to cause turning movement of the wheels of the vehicle, as in the embodiment discussed above.